Multipath fading is a regular phenomenon in telemetry or other communication or location determining operations, especially over water. The arrival of multipath signals at a receiving apparatus can interfere with the reception of the desired, direct path signal. In particular, in a typical multipath situation, a multipath signal is reflected from a surface, such as the surface of the ocean, before reaching the receiver. Because of the longer path traveled by the multipath signal as compared to the direct path signal, the multipath signal may be out of phase with the direct path signal. This can result in destructive interface and attenuation of the direct path signal. Moreover, where the source of the signals and the receiver are in motion relative to one another, the direct path and multipath distances change over time, resulting in a phase relationship that changes according to the difference in target and target image phase.
One standard multipath mitigation technique is to implement a beam tilt. According to this technique, the angle of the receiving antenna relative to the source of the signal is altered. For example, where the receiving antenna comprises a planer array fixed to an aircraft, tilting the beam can comprise altering the attitude of the aircraft from one that is level to one that is non-level. Although this technique can be effective, it is somewhat imprecise, and can be difficult to implement, depending on the flight conditions.
Another technique for mitigating multipath signals involves the use of a relatively large array of antenna elements. In particular, providing an array with a relatively large total aperture, particularly in the vertical dimension, creates spatial independence that can minimize fading issues. However, for reasons including aerodynamic efficiency and weight, there is a desire to reduce the size of receiving antennas. The desire to reduce the size of receiving antennas is particularly strong with respect to the vertical dimension of the antennas, especially in applications where the receiving antenna is mounted to an aircraft.